Vitreous enameling processes and products



Patented May 19, 1953 VITREOUS ENAMELING PROCESSES AND PRODUCTS Allan E. Chester, Highland Park, Ill., assignor to Poor .& Company, Chicago, Ill., a corporation of Delaware No Drawing. Application April 13, 1949, Serial No. 87,375

6 Claims.

This invention relates to vitreous enamelling and more particularly to new and improved base stocks for vitreous enamels and to a new and improved method of controlling the adherence of vitreous enamels to ferrous metals.

One of the principal problems in the art of vitreous enamelling is to secure proper adherence between the vitreous enamel coating and the base stock to which the enamel is applied without the use of a ground or grip coat of vitreous enamel containing cobalt, manganese and usually nickel. It has been recognized that it would be desirable to employ as the enamel base stock ordinary mild carbon steels (also called low-alloy or "mild alloy steels) containing the usual percentages of carbon, as found, for example, in SAE 1010 steels, SAE 1020 steels, SAE 1030 steels, cold rolled commercial quality sheets and cold rolled drawing quality sheets. (See A. I. S. I. Steel Products Manual, section 11, October 1948 issue, pages 34 and 36.) Thus, SAE 1010 steel, as defined by Metals Handbook, 1948 edition, published by the American Society for Metals, page 307, has an average carbon content between 0.08 and 0.13% by weight; SAE 1020 steel has an average carbon content between 0.18 and 0.23% by weight; SAE 1030 has an average carbon content from 0.28to 0.34% by weight. The attempts which have been made heretofore to use such mild carbon steels as a vitreous enamel base stock have met with little success due to warpage and distortion of the object being enamelled. Warping and distortion have been accelerated, moreover, because of the necessity of applying more than one coat of enamel in order to secure adherence. I

Many efforts have been made both by enamellers and steel manufacturers to overcome the handicap of using a ground coat of vitreous enamel followed by a second or even a third coat of enamel. Special steels have been specifically produced and designed for enamel purposes. The most satisfactory vitreous enamel base stocks have been specially prepared from low carbon steels (containing say, 0.01% to 0.02% by weight of carbon) which are more expensive than many of the ordinary steels.

In my copending applications Serial No. 609,759 filed August 9, 1945, now Patent No. 2.510071, issued June 6, 1950, Serial No. 676,853

filed June 14, 1946, now abandoned, Serial No.

tion Serial No, 689,524 filedAugust 9, 1946, now

2 abandoned I have disclosed various methods of controlling the adherence of vitreous enamels to ferrous metals by processes which involve the treatment of th ferrous metal base stock in several stages with certain chemical treating agents. The exact nature of the chemical reactions that take place during these treatments is not fully known, but, in general, the methods which I have employed involve: (1) surface etching of the ferrous metal base stock in the presence of certain types of chemical agents, and (2) applying to the etched surface of said base stock, preferably after rinsing but before drying, a

layer of a metal, or a compound of a metal reducible to the metal, which is reducing with respect to ferrous iron under the vitreous enamel firing conditions (and hence is below ferrous iron in the electrochemical series) and does not flash off or volatilize rapidly at vitreous enamelling temperatures.

One of the objects of the present invention is to provide improvements in the aforementioned methods which contribute to new and improved overall results.

Another object of this invention is to provide 7 a new and improved vitreous enamel base stock.

A further object is to provide a new and im-' proved method of controlling the adherence of vitreous enamels to ferrous metal base stocks.

A still further object of the invention is to produce a satisfactory vitreous enamel article in white and pastel colors with but one coat of enamel.

Another object is to provide a simple relatively inexpensive method of treating ordinary mild carbon steel to increase the adherence of vitreous enamels thereto. Other objects will appear hereinafter. a

In accordance with the invention it has been found that new and improved results are obtained in processes of the character described by the employment of certain procedures and one or more addition agents as follows:

1. The addition of a substance having an electrode potential above ferrous iron in the electromotive force series to a surface etching bath in the treatment of ferrous metals followed by the application of acoating of a metal which is below ferrous iron in the electrochemical series, preferably nickel, cobalt or alloys of nickel and cobalt.

2. The employment of an intermediate treatment after the surface etching stage but before the application of the coating of a metal which is below ferrous iron in the electrochemical series thereby to establish a deposition pattern on the ferrous base metal surface. This intermediate treatment is preferably accomplished by the use of a bath containing dilute sulfuric acid and a small amount of a sulfate of the metal to be deposited or of a metal which i higher in the electrochemical series than the metal to be deposited. For example, if the metal to be deposited in the third stage were nickel or an alloy of cobalt and nickel, the intermediate bath might consist essentially of a dilute (.05 to 3% by weight) solution of sulfuric acid together with nickel sulfate (1 to 28 grams per liter) and/or cobalt sulfate (chemically eguiyalent amounts to nickel sulfate The resultant process, therefore, is necessarily a three-stage process.

3. The maintenance of a ferrous sulfate concentration in the plating bath within predetermined limits, preferably not higher than 10 grams per liter.

4=- T a n enanc 9f hi h? oncen a ion of u ul a by em loyi in the Plating b th ch mi ch s u ters fe ou su at 9 inh ts t cip a io fr solution Citric an is e bly emp o ed for this pu pose a1.-

.th h f m c d nd one s bs anc abl of functioning in a similar manner may also be us d.

5. The use of a metal which does not deposit on the ferrous iron but serves as a driver electromotive force to cause ion intBlGhfiIl e between the iron and other metals. For example, in the surface etching bath where the manganese has b n ad d as ma an se d o ide it a tuafly x sts in solution s ma anese su fa e Th ferrous ion is oxidized to the ferric ion. Zinc in t e m of it su te in this ba h h s a hi h we Pot nt a and ct a a dri er ele tr m i' for for on in e cha ge between the i on an the manganese. Again in the plating bath where, f r sample, n ckel i to b elated. the z nc acts s a ca al st e se i as. a hi her ov rlt than ni l in r o to iron- I a sim ar ma ner cadmium, aluminum and/or magnesium in the f of the solu l sa ts ma e em l n p e of 211 he quan ities. use a Pr f lably 0.1 to 1 grain PS3 liter of zinc sulfate (ZI 1 S,O4.7I-I20) v V or he ch m lly e ui a en mount f t 0 1 meta s lts men ioned- 5, The. employment of a between and 5.5 in the surface etchin bath,

The ment o a p eithe bet een 1- and 3.5 or between 4.5 and 5.5 in a nickel plating bat or n a p a n ba h Where an al oy o co al and nickel are plated.

.li i it d, b he ellowies esei ule n w c h qha i s ar stat d 3 P rts 9-?" Weight ih s XA BLE I Sheets of SAE 1010 and see 102 cold reduced steel of varying gauges (18, 20, 24 and 26 gauge) are etched in a bath composed of the following ngr di n s -35 grams ferrous sulfate (FleSOgHHzO) 5 grams zinc sulfate (ZnSOrJZI-IzO) 0.8 gram manganese dioxide 9,.2 grams sodium bisulfate (NallSQ figO) 3. @cm em l su uric e d Ba- 1 gram isopropylnaphthalene sodium. sulfonate (Santomerse No. l

' m de .1.: to 2 gall ns with w erpickler.

The steel sheets are immersed in. the bath at temperatures of degrees F. to 1'20 degrees F., preferably at degrees F. to degrees F.

The treatment in this first bath is preferably carried out for about 15 minutes, Although the time of treatment may yary over a wider range, it is preferably within the range of 12 to 25 minutes.

The treated sheets are withdrawn from the first bath and immersed in a second bath containing 05% to .3% by weight of sulfuric acid in water and a small amount of nickel sulfate, preferably about 1 gram to 28 grams of nickel sulfate per liter of solution. The temperature is preferably 60 degrees F. to 100 degrees F. Instead of niclzel sulfate, chemically equivalent amounts of cobalt sulfate may be used.

The metal sheets are taken from the second bath and immersed in a third bath having the follow n om os n:

-7 ams nickel ulfa e K wi-6 0 40 grams nickel chloride (NiClzfiI-lzO) 17 gra ns boric acid (H2303) 3.8 grams citric acid 7.0 grams zinc sulfate (ZHSQQJHQQ) 0.3 gram isoprepylnaphthalene sodiimi sulfcnate (Aerosol OS) made up to 2 gallons with water.

The of this bath is maintained around 1.8 to 3.5, preferably around 2.5 to 3.5. The steel sheets are immersed in this bath for periods within therange from 5 to 15 minutes. The temperature of this bath is preferably within the range of degrees F. to 200 degrees F. They are then removed from the bath, rinsed, scrubbed with water and dried, and are ready to be sprayed with suitable vitreous enamel compositions, dried and fired. It is preferable to employ vitreous enamel compositions which can be fired at temperatures not higher than about 1500 degrees F.

EXAMPLE II Steel sheets (22 gauge) were treated on a. large scale as described'in Example I using a Mesta The raw steel was loaded and then immersed in the successive baths. Each bath consisted of 4000 gallons of solution made up in the same relative proportions given in Example I. After the treatment in the third bath, the steel was unloaded, scrubbed, dried and stacked.

In order to compensate for losses of the chemical addition agents of the bath due to their adherence or reaction with the metal and drag-out losses, additional quantities were added by increments to the first bath at the rate of 0.8 pound to 1.74 pounds per ton of steel produced of a composition made by fusing together:

-5 parts @QSQUHQQ 1-0 Par MnQz 1-5 par- NQHS QH A 0.3% sulfuric acid solution was added to the second bath continuously together with 3.75 pounds of NiSO4-6H2O per ton of steel sheet. This second bath was allowed to run over continuously at the rate of 750 gallons per hour. Instead ofthe nickel sulfate, cobalt sulfate was also used.

In the third bath, additional zinc sulfate (ZnS 'O 'ZI-EO) was added in increments at the rate of .yifl to .51 pound per ton of steel produced. The citric acid content was maintained at about 1 gram per liter. Nickel chloride (NiC12.6H2O) was added at the rate of .17; to .51 pound per ton the present case.

example, the steel stripped, the sheet exhibits a "or steel produced. Nickel sulfate (NiSO4.6H2O) .was added at the rate of 8.3 to 9.9 pounds per activity of the bath. If the amount of ferrous iron becomes too high in the third bath, a spongy deposit of iron is formed on the sheet which interferes with the bonding power of the subsequently applied vitreous enamel. The citric acid permits the operation of this bath over a longer period of time by preventing the precipitation of ferrous sulfate. Where no citric acid was used,

the practical limit of ferrous iron concentration was grams per liter, while with the employment of citric acid, this limit may be increased to 30 grams per liter or even higher in some cases to 45 grams per liter, but care must be exercised toinsure that the concentration is below the point at which a spongy deposit of iron occurs.

The surface tension of the third bath is preferably maintained at 80 drops per minute or greater as measured on a Stalagmometer. The pH of this bath may be around 2.0 to 2.5 but may vary over a wider range of 1.8 to 3.5. At a pH around 4.0 there is a very sharp drop in nickel deposition. At a pH of 4.5 to 5.5, however, excellent results are obtained with proper buffers. The quantity of citric acid employed should not be any more than is employed to keep the iron in solution or to prevent precipitation of the ferrous sulfate in the above mentioned operating range. One gram of citric acid per liter of solution will usually be sufficient. Too much citric acid interferes with nickel deposition where the nickel is being deposited by over-voltage as in Formic acid and other substances which have a sequestering or an inhibiting effect on the precipitation of ferrous sul- Ifate may be employed instead of the citric acid.

By a procedure of the type described in this is given a surface etch and manganese salts are deposited thereon and sealed with a nickel plate of approximately 0.01174 to 0.01267 ounce per square foot. The thickness of this nickel plate in terms of millionths of an inch is of the order of 8.0 to 8.6. The sheet containthe nickel coating has a bright metallic finish. The nickel coating may be removed readily with nitric acid solution and after this has been dark grain etch.

EXAMPLE III I Ferrous sulfate is added to the first or surface etching bath to condition the bath. The fering rous sulfate, however, tends to build up in the bath as successive quantities of material are passed through or immersed in the bath. As the ferrous sulfate content of the bath increases, the rateof activation of the ferrous base metal by the etching bath becomes slower and it is preferred for the purpose of this invention to maintain a ferrous sulfate concentration not greater than about two pounds of FeSOMI-IZO per gallon in the etching bath. The following example illustrates the manner in which the ferrous sulfate concentration increases.

In an etching bath such as described in Exbegin with was 17.5 grams per 90.8 grams per gallon of FeSO4.7H2O.

Eight lifts later the temperature of the bath was still 165 degrees F. and an analysis showed 5.0% sulfuric acid by weight and 0.25 pound of FeSO4.'7I-IzO per gallon.

Four lifts later the sulfuric acid content of thebath was 5.5% by weight and the bath contained .32 pound per gallon of FeSOsHHzO.

Eight lifts later the bath analysis was 5.5% sulfuric acid and .37 pound per gallon of The pH of the bath was held between 5.0 and 5.5. The tank was finally dumped when the concentration of FeSO4.7H2O reached 2 pounds per gallon.

Ten pounds of a make-up composition consisting of a fused mixture made by heating 7.5 parts of commercial zinc sulfate, 1.0 part of manganese dioxide and 11.5 parts of commercial sodium bisulfate was added to this bath every fifth lift. In this process, the nickel chloride in the third bath functions to increase conductivity. The boric acid functions as a buffer to maintain the DH.

I The zinc is representative of a metal which furnishes driver electromotive force for the deposition of the nickel in that it has a higher overvoltage than nickel in relation to iron. Cobalt. cadmium, aluminum and/or magnesium may be similarly employed, and inasmuch as the baths contain a sulfate anion such metals will be present as the sulfate.

EXAlWPLE IV over-voltage in the presence of a cathode depolarizer.

Ferrous metal base sheets similar to those used in Examples I, II, and III, but in the form of test plates 1% by 4" (1 2 sq. ft.) were immersed 1n successive baths in three stages.

The first bath had the same composition as the first bath described in Example I except that it contained 205 cc. of 60 degree Baum commercial sulfuric acid. Fifty milligrams of ferrous sulfide were added. The ferrous sulfide decomposes, liberates hydrogen sulfide which assists in activating the sheet. Instead of ferrous sulfide, sodium polysulfide may be used. The temperature'of the bath was maintained at to degrees F. The sheets were treated in 15 to 20 minute cycles, that is, they were immersed for that length of time.

After the treatment in the first bath each sheet was rinsed and immersed in a second bath simestablishes the receptivity or deposition pattern of the sheet; The surface of the sheet may be regarded as containing myriads of couples with thesubsequent deposition in the third bath occurring only on those areas of the proper polarity. The same time period was employed in the second bath, that is, 15 to 20 minutes.

aaeaacs he third ba consisted of tbcpt l cwing in re'dientsz. a

The PH of his ba h w s maintain d a 1.9 o 5.1 and the treatment again was in a to minute cycle. In this bath the use of the dextrose is optional. The compound derived by the reaction of the sodium hydroxide, chomic acid and gluconic acid acts as a depolarizer. The sodium bisulfite functions as a reducing agent. This bath deposits alloys of nickel and cobalt by over-'voltage onto the ierrous base stock pre viously treated in the first and second baths. For the purpose of this invention, excellent results have been obtained where the allow plated consisted essentially of 90% niclgeiand 10% cobait or 92.8% nickel and coba In the foregoing examples, cold rolled commercial quality and drawing quality sheets may be used with excellent r sults. One u h steel is specified as .08 carbon ma and 52 0 4 m ganese, as described in A. I. S. I. Steel Products Manual, sec. 11, October 19%8, pp. 34 and 36.

The term mild alloy steels as used herein is intended to cover steels having an average car.- bon content in excess of 0.02% and not more h n M502 preferably 0.07 to 0.20%, lud the steels having the iollowing specific compositien-s:

Compositions 0 Mn Si P Ni Cr Cu M0 00- Low 55 .005 45 24 0.10 0. 25 0. 75 0.15 0. 40 0.12 0. so 0.08 0. 40 0. 08 0. 0. 95 0. 0.15 0. 70 0.17 0. e0 0. 10 0. 00 0.15 0.12 1.10 0.11 0. 0 0.15 0.30 0.10 0. s0 0.10 0. 00 0.10 0. 35 0.08 0.25 0.13 0. 00 0.15 0.20 0.10 0. 75 o. 30 0.10 0. 0.13 0. 70 0.80 0.20 (Zr =0 12 0.10 1.10 0.05 0.12 0. 35 0.12 0.15 1.40 0.10 0.15 1. 00 0.50 0. 25 0.10 0.08 0.50 0.25 1.00 0. 07 0.35 Trace 0. 00 0. 4

Th employment of the variou addition a n s, s ch th sulfate to supply driver electromotive force, the citric acid to prevent precipitation of the ferrous sulfate, the nickel chloride to increase conductivity, the buffer materials to maintain the pH and the wetting agents to reduce surface tension and to overcome gas polarization, all represent optional procedures which can be emp oye n conjunction with t anganese dioxide activation or with other treat.- ments of the type described in my aforementioned ccpending applications. Each of these types of addition agents contributes to a very substantial m ro ent in the overall resu ts.

metal hich-i to be treated may b a r gular co d rolled box annealed steel sh et o the ype mmonly referred to as autobody sheets.

Th -con rol 0f the compositi nal themetalt hc nne l o t sheet a d h temper rolling, if any, wil be d t m ed f om t r quiremen s of the end use 01? the sheet, especially as to the amount of drawing and firing which has to be done. Temper rolling of the sheet may be performed before or after the processing herein described, whichever is more convenient in a particular plant.

- As an example of a suitable vitreous enamel om os ti there m y b entioned a molybdenum bearing enamel which is made from the following smelter batch:

Parts by weight Boraxhydrous 21.3 Silica sand 31.8 Soda ash 2.9 Potassium nitrate 5.5 Sodium fiuosilicate 5.3 Fluorspar 2.2 Barium carbonate 8.3 Zinc oxide 3.4.- Titanium dioxide 4.5 Molybdic oxide-pure 4.0 Sodium antimonate 10.8

The finishing and pouring temperature of this batch after smelting is approximately 2100 degrees F.

For coats applied by spraying the following mill batch may be used:

Frit (parts by Weight) 1 Clay (parts by frit eight) 4 Eodium nitrite (parts by .frit weight) 0.5 Antimony opacif er (parts by frit weight) 4 Water (parts by Irit weight) 40 Parts by weight NazSiFe 22.6

A typical feldspathic frit is made from the following ingredients:

Parts by weight Feldspar 21.7. Borax 31.0 Quartz 7.0 Soda ash 2.4 Barium carbonate 8.6 Sodium nitrate 5.7 Zinc oxide 7.0 Fluorspar 7.6 Whiting m Sodium zirconium silicate 1,, L5

. The forego ng ri are made the u al m 11 1 by 1.0 8 31%. smel ing. hittin and drying the c m sitions The frits are then mixed together in the 60:40] ratio and the following mill additions are made:

Frit (parts by weight) 100 White clay (parts by frit weight) 7 Color oxide or opacifier (parts by frit weight) -20 Bentonite (parts by frit weight) /8 Barium carbonate (parts by frit weight) A1 Water (parts by frit weight) 43 The resultant composition is milled to a fineness of L2 grams residue on a 200 mesh screen from a 50 cc. sample and then applied to the previously prepared ferrous metal base stock, dried and fired at a temperature within the range of 1250 degrees F. to 1300 degrees F.

Excellent results were obtained on the steel sheets treated as previously described with both types of vitreous enamels mentioned.

The importance of the invention resides in the fact that it provides a very simple and economical method of increasing the adherence of vitreous enamel coatings to ferrous metal base stocks without the use of ground or grip coats of vitreous enamels containing cobalt, manganese and/or nickel. As a result, it is possible to prepare vitreous enamel coated articles containing only a single coat of the vitreous enamel possessing excellent adherence to the base stock. The simplicity of the procedures involved in securing this adherence is such that the enameler is not required to depend upon the use of a special type of base stock. The base stock used can be similar to that now employed in making automobile bodies. Another important advantage of the invention is that it can be applied in the treatment of welded shapes.

The invention is hereby claimed as follows:

1. A method of controlling the adherence of vitreous enamels to mild alloy steels which consists essentially in subjecting a mild alloy steel having a carbon content between 0.02% and 0.20% to the action of a surface etching bath comprising an acid solution of ferrous sulfate, manganese dioxide and a compound from the group consisting of the soluble salts of zinc, cadmium, aluminum and magnesium, treating the steel with a dilute substantially non-etching acid solution containing a soluble salt of a metal from the group consisting of nickel and cobalt, then electrodepositing on the steel a thin coating of a metal from the group consisting of nickel, cobalt and alloys of nickel and cobalt, and firing a vitreous enamel on the plated surface.

2. The product of the method recited in claim 1.

3. The method of controlling the adherence of vitreous enamels to mild alloy steels which consists essentially in subjecting a mild alloy steel having a carbon content between 0.02% and 0.20% to the action of a surface etching bath comprising an acid solution of ferrous sulfate, manganese dioxide and a compound from the group consisting of the soluble salts of zinc, cadmium, aluminum and magnesium, treating the steel with a dilute substantially non-etching acid solution containing a soluble salt of a metal from the group consisting of nickel and cobalt, then electrodepositing on the steel a thin coat-- ing of a metal from the group consisting of nickel, cobalt and alloys of nickel and cobalt, said electrodeposition being carried out in an aqueous electrolyte containing in solution a compound from the group consisting of the soluble salts of zinc, cadmium, aluminum and ma nesium, and firing a vitreous enamel on the plated surface.

4. The product of the method recited in claim 3.

5. A method of controlling the adherence of vitreous enamels to mild alloy steels which consists essentially in subjecting a mild alloy steel having a carbon content between 0.02% and 0.20% to the action of a surface etching bath comprising an acid solution of ferrous sulfate, manganese dioxide, a compound from the group consisting of the soluble salts of zinc, cadmium, aluminum and magnesium, and a buffer capable of maintaining the pH of said bath between 5 and 5.5, treating the steel with a dilute substantially non-etching acid solution containing a soluble salt of a metal from the group consisting of nickel and cobalt, then electrodepositing on the steel a thin coating of a metal from the group consisting of nickel, cobalt and alloys of nickel and cobalt, adding a buffer to the bath from which said thin coating of metal electrodeposited capable of maintaining the pH of the plating bath between 1.8 and 3.5 and firing a vitreous enamel on the plated surface.

6. The product of the method recited in claim 5.

ALLAN E. CHESTER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,837,835 Pinner Dec. 22, 1931 2,032,256 Canfield Feb. 25, 1936 2,101,950 McGohan Dec. 14, 1937 2,127,388 Canfield Aug. 16, 1938 OTHER REFERENCES Gregory: Uses and Applications of Chemicals and Related Materials (Reinhold), 1939, page 191. 

1. A METHOD OF CONTROLLING THE ADHERENCE OF VITREOUS ENAMELS TO MILD ALLOY STEELS WHICH CONSISTS ESSENTIALLY IN SUBJECTING A MILD ALLOY STEEL HAVING A CARBON CONTENT BETWEEN 0.02% AND 0.20% TO THE ACTION OF A SURFACE ETCHING BATH COMPRISING AN ACID SOLUTION OF FERROUS SULFATE, MANGANESE DIOXIDE AND A COMPOUND FROM THE GROUP CONSISTING OF THE SOLUBLE SALTS OF ZINC, CADMIUM, ALUMINUM AND MANGANESIUM, TREATING THE STEEL WITH A DILUTE SUBSTANTIALLY NON-ETCHING ACID SOLUTION CONTAINING A SOLUBLE SALT OF A METAL FROM THE GROUP CONSISTING OF NICKEL AND COBALT, THEN ELECTRODEPOSITING ON THE STEEL A THIN COATING OF A METAL FROM THE GROUP CONSISTING OF NICKEL, COBALT AND ALLOYS OF NICKEL AND COBALT, AND FIRING A VITREOUS ENAMELS ON THE PLATED SURFACE. 